The use of addition polymers such as acrylic polymers in coatings formulations, particularly in topcoats for the automotive market, has been widespread. One of the primary requirements of such a coating for the automotive market is that it be durable. That is, the coating must resist degradation due to environmental elements which it comes into comes into contact with, such as sunlight, gasoline, and environmental fallout such as dew, rain, or particulate matter of any kind. It has been recognized that prior art coating compositions are deficient in their ability to withstand degradation from elements which have generally been lumped together in the category of "environmental" causes of degradation. This sort of degradation has manifested itself in an etching or pitting of the coating's surface, and has resulted in the necessity of the automotive manufacturers to repair the coatings under warranty claims. However, in spite of the recognized deficiencies of prior art polymers and coating compositions, as well as the economic penalty of such deficiencies, a long felt need for such a polymer for producing a cured topcoat resistant to environmental etch has heretofore gone unsatisfied.
In the field of automotive coatings, it has become an objective to obtain a topcoat or clearcoat of a color-plus-clear composite coating that is resistant to being etched by environmental fallout. A color plus clear composite coating refers to a multi-layer coating applied to a surface, particularly an automotive vehicle surface, where at least one pigmented coating is applied to a surface and at least one substantially transparent coating (i.e. clearcoat) is applied over the pigmented coating layer.
Currently automotive clearcoats may comprise one component or two-component coating compositions. One-component compositions include all components in one mixture and react to crosslink at relatively high temperatures in the presence of a catalyst. Two-component compositions comprise two or more reactive solutions or dispersions which react upon contact and therefore must be mixed immediately before being applied to a substrate.
One component compositions that use a melamine crosslinker are widely used, but demonstrate unsatisfactory environmental etch resistance when used in cured automotive clearcoats. Also, melamine systems split off a by-product upon crosslinking. This by-product is usually a low molecular weight organic solvent, which adds to the volatile organic compounds (i.e., VOC's) which are released (or captured) during the coating process. Further, the small organic compounds released may become involved in undesirable reactions with other components in the composition or with components present in adjacent coatings.
One component compositions comprising a blocked isocyanate crosslinking agent also exhibit several disadvantages. First, the presence of the blocking agent produces the need to utilize proportionally more solvent/dispersant, because the blocking agent enhances the size of the crosslinker, requiring more solvent or dispersant. Second, upon deblocking, the blocking agent is volatilized, thus increasing the volatile organic compounds (VOC) present in the coating, when compared to the unblocked two component compositions. Third, the release of the blocking agent in one component composition places a mobile species into the film. The presence of this mobile blocking agent can be a detriment, since it could potentially react with other species present in the film or in an adjacent uncured film layer which may be also present. Fourth, the deblocking agent represents a cost which must be borne by the manufacturer and user of the blocked isocyanate composition. Finally, the use of a blocking agent requires that the composition be brought to a higher temperature than is required for the curing of two component compositions. The use of higher temperatures for the curing operation is undesirable because it requires the input of greater energy (i.e., it is expensive), and because it can result in deformation of plastic automotive body panels during the curing step.
The two-component composition, process, and coating of the present invention are novel in that they achieve excellent etch resistance using an acrylic polymer having a high glass transition temperature (T.sub.g). It is unexpected that a high T.sub.g acrylic based on high T.sub.g monomers would provide good resistance to environmental etch, as such an acrylic would be expected to be very brittle and too fragile for automotive coating application. Further, when a polymer contains a high T.sub.g monomer such as styrene in an amount greater than about 25 weight percent, a detrimental "loss of gloss" (i.e. chalking) may result. Such a loss of gloss is considered to be a form of coating deterioration which is completely different from environmental etching.